The present invention relates to an imaging lens for forming an optical image on an imaging element such as a CCD sensor and a CMOS sensor. Particularly, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.
Performances of a camera mounted in a cellular phone have been improved every year, and there is even a camera having optical performances comparable to those of a digital still camera. The resolution of an imaging element that used to be several hundred thousand pixels has been improved to several megapixels, and the resolution of a camera has been dramatically improved. An imaging lens for mounting in such camera needs to not only have a small size, but also have resolution comparable to that of a high-resolution imaging element, i.e., ability to satisfactorily correct aberrations.
Conventionally, in order to attain both sufficient optical performances and miniaturization, it is typical to adopt an imaging lens having two or three-lens configuration. However, optical performances required for such a high-resolution imaging element have been increased each year, so that it is difficult to sufficiently correct aberrations anymore. Further, it is difficult to meet those optical performance requirements with such a two- or three-lens configuration. For this reason, a four- or five-lens configuration has been studied and practically applied in recent years.
Among them, since a five-lens configuration has high flexibility in designing, it is expected to be a lens configuration to be applied to a next generation imaging lens. For example, an imaging lens described in Patent Reference includes a first lens that is convex on an object-side surface thereof and is positive; a second lens that has a shape of a negative meniscus lens directing a concave surface thereof to an image plane side; a third lens that has a shape of a positive meniscus lens directing a convex surface thereof to the image plane side; a fourth lens that has aspheric shapes on both surfaces thereof, is concave on an image plane-side surface thereof near an optical axis, and is negative; and a fifth lens that has aspheric shapes on both surfaces thereof and is positive or negative, arranged in this order from the object side. In addition, there are limitations in a lower limit of Abbe's number of the first lens and an upper limit of Abbe's number of the second and the fourth lenses. With the lens configuration like this, according to the imaging lens, it is possible to satisfactorily correct an axial chromatic aberration and a chromatic aberration of magnification.
Patent Reference: Japanese Patent Application Publication No. 2007-264180
According to the imaging lens described in Patent Reference, it is possible to obtain relatively satisfactory aberrations. However, since the total length of the lens system is long, it is difficult to attain both miniaturization of the imaging lens and satisfactory aberration correction. Here, such challenge of attaining both miniaturization and satisfactory aberration correction is not specific to the imaging lens for mounting in a cellular phone, but common among imaging lenses for mounting in relatively small cameras such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
In view of the above-described problems, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations in spite of the small size thereof.
Further objects and advantages of the invention will be apparent from the following description of the invention.